Enhanced stiffness combined with superior processability has been a long-standing interest in the polypropylene (PP) industry. Some attempts to achieve this balance include the use of bimodal PP blends, either in situ (in reactor) or ex situ (extruder blends), that include a combination of a relatively low molecular weight polypropylene (or high MFR) and a relatively high molecular weight polypropylene (or low MFR). The challenge is particularly relevant for blends of single-site based PP's (ssPP's), which alone are not easily processable.
In particular, use of a high molecular weight (HMW) PP can lead to improvement in the stiffness of polypropylene blends. However, inclusion of HMW PP in blends may be accompanied by a loss in the processability due to increased viscosity if the properties of the components are not properly balanced. Depending on the ultimate application, solutions to achieve a reasonable balance between stiffness and processability may involve adjusting the properties of the individual polypropylene components and/or addition of some other polymeric additive such as an elastomeric component, either of which may be accompanied by forming bimodal PP blends. While in-reactor methodologies to form bimodal PP blends are well documented, physical blending of two unimodal polypropylenes PP's to realize bimodality has not shown enhanced stiffness and processability, both for single-site (SS) and Ziegler-Natta (ZN) homopolymers.
The present invention describes new bimodal PP compositions prepared by melt blending exhibiting enhanced stiffness and improved rheological properties that translate to superior processability. Both, SS unimodal polypropylenes (ssPP's), as well as ZN-based unimodal PP's currently being utilized in Impact Copolymer (ICP) manufacture have been included as relevant comparisons to demonstrate this improvement. Besides offering an additional pathway to achieve these objectives, physical blending can be easily implemented and practiced using commercial extruders.
Relevant references include: U.S. Pat. Nos. 5,744,548; 6,747,103; 6,809,168; 6,087,750; 6,350,828; 7,319,122; EP 1 801 155; EP 2 386 601; A. Elmoumni et al. in “Isotactic Poly(propylene) Crystallization: Role of Small Fractions of High or Low Molecular Weight Polymer,” 26 MACROMOL. CHEM. PHYS. 125-134 (2005); M. Seki et al. in “Shear-Mediated Crystallization of Isotactic Polypropylene: The Role of Long Chain-Long Chain Overlap,” 35 MACROMOLECULES 2583-2594 (2002); W. Minoshima et al. in “Experimental Investigations of the Influence of Molecular Weight Distribution on the Rheological Properties of Polypropylene Melts,” 20(17) POLY. ENG. & SCI. 1166 (1980); B. Rabinowitsch, A145 Z. PHYSIK. CHEM. 1 (1929); 35 MACROMOLECULES 3838-3843 (2002); 46 RHEOL. ACTA 33-44 (2006); 44 POLY. SCI. AND ENG. 2090-2100 (2004); and MACROMOL. SYMP. 46-52, 321-322 (2012).